A plate glass manufactured by cooling a molten glass plate according to a conventional process carries residual internal stress which causes distortion when the plate is cut into a desired size. Such a cut plate when used as a display glass panel exhibits poor picture display characteristics.
Accordingly, in order to eliminate the internal stress remaining in a plate glass, Japanese Publication Patent Nos. 1993-306133, 2000-86261 and 2001-322823, and European Publication Patent No. 832,858 disclose a method of subjecting a cut glass plate to annealing post-treatment. However, such an annealing process has problems in that it requires a prolonged processing time and the annealed glass surface needs polishing to correct the surface deformation brought about by the annealing process.
The internal stress remaining in a glass plate after cooling a molten form thereof arises from the viscoelastic properties of glass, and when the theory and experimental information developed by Narayanaswami et al. are applied to the plate glass cooling process, only the residual stress across the thickness thereof can be assessed. However, such distortion of a plate glass occurs mainly along the horizontal width direction of the plate because of the significant temperature difference between the ends and center of the plate during cooling.
Accordingly, the present inventor has endeavored to develop a method for reducing such residual stress formed along the horizontal width direction of the plate and has found that a flat glass plate having a low residual stress can be prepared by adjusting the horizontal temperature gradient of a molten glass plate to a specific range during cooling.